1. Field of the Invention
The present invention relates to a trunk network for realizing multipoint-to-multipoint relay, and more particularly to the notification of failures in a trunk network.
2. Description of the Related Art
Networks exist that are provided with alternate routes for improving reliability against failures. In the event of a failure on a transmission route in a network that is provided with alternate routes, the transmission of packets can be continued by switching to an alternate route. In such a case, however, communication is dropped during the time interval from the occurrence of the failure until the completion of switching.
As a result, shortening the time interval from the occurrence of the failure until switching is completed (hereinbelow referred to as the “route switching time interval”) both obtains a corresponding shortening of the time interval in which communication is dropped and an improvement in the reliability of the network. The route switching time interval includes the time interval required from the occurrence of the failure until its detection (hereinbelow referred to as the “failure detection time interval”), and this failure detection time interval is a crucial factor in determining the reliability of the network.
Trunk networks exist that have a packet switching capability that provides multipoint-to-multipoint connection, these trunk networks providing connections between a plurality of external devices such as routers or MAC switches. Through the use of this type of trunk network, external devices are connected each other by way of a plurality of nodes.
If a failure should occur in this type of trunk network, the reachability of packets that pass through the trunk network can no longer be guaranteed. Examples of failures that would impede the reachability of packets include the failure of an external device, the failure of a port of a relay node that connects with an external device, or the failure of a relay node.
A device that is directly connected to the site of a failure can detect a device failure or a port failure as a link down (disconnection). By comparison, an external device that is not directly connected with the site of a failure cannot directly detect the failure.
A device that provides point-to-point connection, upon detecting that one link is down, can cause disconnection of the other link and thus enable notification of a failure to a device that cannot directly detect a link that is down (for example, refer to JP-A-2003-087276)
On the other hand, there are methods in which, as a method for verifying packet reachability in a trunk network that provides multipoint-to-multipoint connection, i.e., as a method for detecting failures, external devices exchange with each other control packets for survival verification. The external devices continuously send control packets at a prescribed period to partner devices, and further, monitor whether control packets are being continuously received from partner devices. The external devices then determine that reachability has been impeded when the control packets are no longer received and thus switch the transmission route of the packets.
However, the above-described background art has the following problems:
A method in which the disconnection of one link precipitates the disconnection of the other link in a device having the above-described type of point-to-point connection cannot be adapted to a trunk network that provides multipoint-to-multipoint connection. This inability comes as a result of the lack of a one-to-one correspondence between links in a trunk network that provides multipoint-to-multipoint connection. In other words, when the disconnection of a particular link is detected, determining which other link is to be disconnected is problematic because various connections are possible between links in a multipoint trunk network.
As a result, each external device preferably verifies by some method whether the reachability of packets by way, of the trunk network is secured or not, i.e., whether a failure has occurred or not. As an example, there is a method in which the above-described control packets for verifying survival are exchanged.
This method, however, generally entails a long time interval for detecting a failure. Shortening the period for transmitting control packets can shorten the failure detection time interval. However, shortening the period for transmitting control packets tends to increase the processing load in external devices or place pressure on the communication bandwidth of the transmission line. Consequently, there is typically a limit to shortening the transmission period of the control packets, and adequately shortening the route switching time is therefore problematic. A method realized by means of survival verification control packets typically requires a failure detection time interval ranging from ten seconds to several minutes.
As a result, during the long time interval from the occurrence of a failure until completion of switching, the external device continues to transmit packets to the route in which reachability has been impeded, lengthening the time interval in which communication is dropped.